Attempts have been made to adjust a path of a sound wave before emitted from an outlet opening in a speaker system. For example, in a sound wave guide path formed around an internal element provided inside a housing having an outlet opening of a slit shape, all shortest paths extending from an inlet opening to the outlet opening are configured to have a substantially equal length. Thereby, the sound wave is emitted from the outlet opening entirely in isophase to form a wavefront (isophase plane) of a rectangular planar shape (see e.g., specification of U.S. Pat. No. 5,163,167).
However, since it is difficult to design the sound wave guide path so that the wavefront of the emitted sound wave has shapes other than a rectangle, for example, a concave curved plane shape or a convex curved plane shape, and it is necessary to provide the internal element, the number of components increases and a manufacturing step becomes complicated. Furthermore, such a structure is intricate.